It has been known that cellulose derivatives inclusive of hydroxypropyl cellulose (HPC) and cellulose triphenylcarbamate form chiral nematic (cholesteric) liquid crystalline phases by being dissolved in a suitable solvent under appropriate conditions, e.g., at an appropriate temperature or concentration. The structure of a chiral nematic liquid crystal is described in detail in, for example, Gray, Carbohydrate Polymers, 25, 277-284 (1994). The chiral nematic liquid crystal has a structure where sheet-like nematic liquid crystalline phases are helicoidally piled with the orientations thereof rotated from layer to layer. The chiral nematic liquid crystalline layers has a peculiar optical property that, defining a wavelength corresponding to the product of the pitch and the average refractive index as the maximum, the pitch being the distance taken by the orientation of the sheet-like nematic liquid crystal to rotate through 360 degrees, it selectively reflects circularly polarized light corresponding to the handedness of the sheet-like nematic liquid crystal. It is reported in detail, for example, that such selective reflectivity of the chiral nematic liquid crystal is useful for laser systems as a part of a circularly polarized light generator [see, for instance, Jacobs, Journal of Fusion Energy, 5(1), 65-75 (1986)]. When the wavelength of light selectively reflected by a chiral nematic liquid crystal lies within the visible region, the chiral nematic liquid crystal presents a beautiful color, thus its application to, e.g., decoration materials is also desired.
Generally, since the formation of a chiral nematic liquid crystal is sensitive to conditions such as temperature and concentration and a chiral nematic liquid crystal phase in a liquid state has to be enclosed or sealed in a gap between the glass plates, its use or applications have been limited and its industrial use is rare.
To solve the above-mentioned problems, there have been made attempts to solidify a polymer solution containing a chiral nematic liquid crystal phase for fixing its liquid crystalline structure. For example, in the Journal of Applied Polymer Science,49, 125(1993), there is disclosed a technique comprising the steps of dissolving an ethyl-cyanoethyl cellulose in acrylic acid to form a chiral nematic liquid crystal and then photopolyimerizing the acryic acid contained in the liquid crystal solution to fix its liquid crystalline structure. The technique disclosed by this report is a pioneer one among the techniques for fixing a liquid crystalline structure. However, considering that the synthesis of ethyl-cyanoethyl cellulose requires the use of an acrylonitrile which is thought of as a potential carcinogen, industrially producing ethyl-cyanoethyl cellulose is practically difficult. The literature says that a fixed chiral nematic liquid crystal presents a color due to its selective reflectivity, but no data on its optical properties are disclosed.
In the Polymer Journal, 17,753 (1985), there is disclosed a technique in which a commercially available ethyl cellulose with a degree of substitution of about 2.5 is dissolved in acrylic acid to form a chiral nematic liquid crystal and the liquid crystal solution is then thermally polymerized. This literature, however, does not provide a solid shaped article having selective reflectivity. Moreover, even if a solid shaped article having selective reflectivity is obtained from a chiral nematic liquid crystal solution comprising an ethyl celluose and acrylic acid by adopting the photopolymerization technology disclosed by the aforementioned literature [Journal of Applied Polymer Science, 49, 125 (1993)], the liquid crystallinity of the article is still insufficient and consequently birefringence and selective reflectivity are poor.
In Japanese Patent Application Laid-Open No. 25365/1998, there is disclosed a technique for fixing a liquid crystalline structure by dissolving a cellulose phenylurethane in a polymerizable solvent to form a chiral nematic liquid crystal and polymerizing the polymerizable solvent contained in the liquid crystalline solution. The technique disclosed by this literature is effective in obtaining a polymeric shaped article in a solid state without losing the optical properties of the chiral nematic liquid crystal. In many cases, however, this technique requires the introduction of a phenylcarbamate group into the cellulose and, in many cases, the introduction of a halogen atom such as chlorine or fluorine into a phenyl group for adjusting the selective reflection wavelength to the desired value. Accordingly, there arise problems of harmful effects on the environment that might be caused by the substances by-produced or wasted in the production of cellulose urethanes. Moreover, a cellulose phenylurethane is generally prepared by reacting a cellulose with a phenylisocyanate. However, the synthesis of a phenylisocyanate usually involves the use of phosgene of strong toxicity and therefore unfavorable in view of environmental protection. For such reasons, industrial production of cellulose urethanes are scarcely conducted. Even if brought to industrial production, the cost is high.
In the above-mentioned patent specification, cellulose ethers and cellulose esters are also listed as cellulose derivatives. However, a mixture of a cellulose acetate with a polymerizable monomer does not show liquid crystallinity. As can be understood from the above, a theory which is substantially valid and effective in determining conditions and solvents appropriate for forming a liquid crystalline phase, including the species of substituents or degree of substitution of cellulose derivatives, has not been found yet. To selectively reflect light of the desired wavelength, the pitch of the liquid crystalline phase should be controlled within the range of about 150 to 2,000 nm, and a theory substantially valid and effective for the control of the selective reflection wavelength and the pitch has also not been established yet.